At present, examples of practical secondary battery systems mainly include a lead-acid battery, a nickel-hydrogen battery, and a lithium ion battery. These batteries play important roles in our daily life and socioeconomic development. The use of a lead-acid battery is, however, limited due to the low energy density thereof and the serious pollution to the environment. In the past few years, a nickel-hydrogen battery has been a mainstream battery in hybrid systems due to the mature technology and high safety. A nickel-hydrogen battery has, however, an insufficient energy density, with a small room for further improvement in performance because of a limit being reached at which its technological development is no longer possible. Since a secondary lithium battery has advantages such as a high energy density, a long cycle life, and environmental friendliness, the research and development thereof is receiving attention. The use of metal lithium is currently limited only to an intercalation negative electrode material such as graphite, due to high activity with a low melting point and the occurrence of phenomenon of deposition of dendritic lithium crystals in many of organic electrolytic solutions. In addition, a lithium secondary battery has unsolved reliability and safety concern. The electrochemical research is therefore directed to the development of a new-type battery having a high specific energy and high safety, causing no pollution.
Metal magnesium as a negative electrode has a theoretical specific capacity of 2205 mAh/g, and an electrode potential of approximately −2.37 V vs. SHE, with excellent conductivity and mechanical properties. Magnesium in particular has advantages such as a low cost (approximately 1/24 of the cost of lithium), high safety, and environmental friendliness. A magnesium battery is thus excellent in safety and cost. Researches on magnesium as electrode material thus draw attention of scientific researchers (The Journal of The Electrochemical Society, 1990, 137 (3): 775-780). According to the research results of magnesium secondary battery by researchers such as Aurbach, it is believed that although a magnesium secondary battery cannot be competitive with a lithium ion battery in the field of small-sized appliances (e.g. portable electric appliances), it has potential advantages for use in large-sized appliances, being applicable as a green rechargeable battery for use in an electric vehicle or an energy accumulation device (Nature, 2000, 407 (6805): 724-727).
A main factor for limiting the development of a magnesium secondary battery is the difference in characteristics between an inactivated membrane formed on the surface of metal lithium and an inactivated film formed on the surface of magnesium in many aprotic electrolytic solutions. The inactivated membrane formed on the surface of lithium is a good conductor of lithium ions, while the inactivated membrane formed on the surface of magnesium is a poor conductor of magnesium ions. Since magnesium ions thus cannot pass through the inactivated membrane, the electrochemical activity thereof is restricted. It can be said that the development of a magnesium secondary battery is greatly related to the development of an electrolytic solution.
According to the past research results, the reversible deposition of magnesium has not been achieved in an aprotic polar solvent of a simple ionized magnesium salt (e.g. MgCl2, Mg(CLO4)2, and Mg(CF3SO3)2) (Journal of Electroanalytical Chemistry, 1999, 466 (2): 203-217). The reversible deposition and elution of Mg can be achieved in an ether solution of a Grignard reagent. A common Grignard reagent, however, has a narrow electrochemical window and high activity, so that it cannot be directly used as a magnesium secondary battery electrolytic solution. In 1990, Gregory et al. disclosed that an ether solution of Mg[B(Bu2Ph2)]2 also allowed for the reversible deposition of Mg, wherein Eu represents butyl and Ph represents phenyl (U.S. Pat. No. 4,894,302). It has an electrochemical window at approximately 1.9 V (vs. Mg/Mg2+), which is much higher than that of a common Grignard reagent by several hundred mV. A magnesium secondary battery Mg∥ 0.25 mol/L Mg[B(Bu2Ph2)]2/70% THF+30% DME∥ Co2O4 was assembled for the first time, using the electrolytic solution. The battery system had a utilization factor of the positive electrode active material of 86% and a coulombic efficiency of charging/discharging of 99%. Although the battery had a low discharging voltage and dreadful polarization, it demonstrated that a magnesium secondary battery was technically viable.
Problems to be Solved by the Invention
At present, the most mature example of magnesium secondary battery electrolytic solution systems is formed of 0.25 mol/L Mg(AlCl2EtBu2)/tetrahydrofuran, wherein Et represents ethyl and Bu represents butyl, which was proposed in 2000 by Aurbach who is a scientist in Israel. The electrolytic solution having a stable electrochemical window at 2.2 V vs. Mg/Mg2+ or higher allowed a big step toward practical application of magnesium secondary batteries to be taken (Nature, 2000, 407: 724-727). The electrolytic solution, however, has a relatively narrow electrochemical window, so that the use of a positive electrode material having a high redox potential and capacity is restricted. The electrolyte system is thus the most important bottleneck in the development of magnesium secondary batteries.
In recent years, polymer solid electrolytes of a magnesium battery have attracted attention, and various types of polymer systems have been presented (Solid State Ionics, 2000, 128 (1-4): 203-210; and Journal of Power Sources, 2001, 102 (1-2): 46-54). An additive such as SiO2 nano particles, magnesium salt nano particles, or an ionic liquid has been added to the polymer systems. The research content, however, basically has included the measurements of conductivity and cycle voltage-current curve only, not relating to the reversibility in electrode processes such as magnesium deposition/elution, lacking the direct evidence of metal magnesium deposition. As a result, no polymer solid electrolyte system for use in a secondary magnesium battery has been disclosed. In order to develop a polymer electrolyte of a magnesium battery, it is necessary to understand the ionic dissociation and conductivity mechanism in an electrolyte for the research of electrical deposition process of metal magnesium. On the other hand, an ionic liquid has advantages such as hardly causing volatilization and thermal decomposition and having high thermal stability and a wide electrochemical window, to satisfy the conditions required for the solvent of a magnesium secondary battery. Accordingly, researches on an ionic liquid have increased in recent several years. Since the interface characteristics of an ionic liquid electrolytic solution are complicated and dominantly affected by the purity of an electrolytic solution, the efficiency of reversible deposition and the cycle stability of magnesium are not ideal yet at present. For the development of a high-performance magnesium secondary battery, the development of an electrolytic solution system having a higher conductivity and anode oxidation potential, a high efficiency of reversible deposition/elution of magnesium, and excellent cycle performance is in the main direction of development of the current magnesium secondary battery.
The present invention has been made to solve the above technological problems. An object of the present invention is to provide a new-type magnesium secondary battery electrolytic solution having a wide and stable electrochemical window and excellent reversible deposition characteristics of magnesium.